IndexOutOfBoundsException in BiRandomMoveIterator when using custom neighborhood move

[lshlmy]

I'm getting an IndexOutOfBoundsException when using a custom neighborhood move in Timefold Solver. The error occurs during the local search phase inside BiRandomMoveIterator.pickNextMove().

Stack trace:
java.lang.IndexOutOfBoundsException: The index (50) must be >= 0 and < size (50). at ai.timefold.solver.core.impl.util.ElementAwareArrayList.getEntry(ElementAwareArrayList.java:66) at ai.timefold.solver.core.impl.util.ElementAwareArrayList.get(ElementAwareArrayList.java:112) at ai.timefold.solver.core.impl.bavet.common.index.DefaultUniqueRandomIterator.hasNext(DefaultUniqueRandomIterator.java:62) at ai.timefold.solver.core.impl.bavet.common.index.FilteredUniqueRandomIterator.hasNext(FilteredUniqueRandomIterator.java:40) at ai.timefold.solver.core.impl.neighborhood.stream.BiRandomMoveIterator.pickNextMove(BiRandomMoveIterator.java:109) at ai.timefold.solver.core.impl.neighborhood.stream.BiRandomMoveIterator.hasNext(BiRandomMoveIterator.java:85) ... at ai.timefold.solver.core.impl.localsearch.DefaultLocalSearchPhase.solve(...)

Version:
Timefold Solver 2.1

Configuration:
I am using a custom neighborhood move selector similar to:
`

NEIGHBORHOODS

<solutionClass>com.logistics.delivery.vrp.optaplanner.model.VrpSolution</solutionClass>
<entityClass>com.logistics.delivery.vrp.optaplanner.model.VrpVisit</entityClass>
<entityClass>com.logistics.delivery.vrp.optaplanner.model.VrpVehicle</entityClass>

<scoreDirectorFactory>
    <constraintProviderClass>com.logistics.delivery.vrp.optaplanner.constraint.provider.VrpConstraintProvider</constraintProviderClass>
</scoreDirectorFactory>



<termination>
    <secondsSpentLimit>120</secondsSpentLimit>
</termination>


<!-- 阶段一：禁忌搜索修复硬约束 -->
<localSearch>
    <termination>
        <secondsSpentLimit>48</secondsSpentLimit>
        <unimprovedStepCountLimit>500</unimprovedStepCountLimit>
    </termination>

    <unionMoveSelector>
        <listChangeMoveSelector/>
        <listSwapMoveSelector/>
        <subListChangeMoveSelector/>
        <subListSwapMoveSelector/>
        <kOptListMoveSelector/>

        <changeMoveSelector>
            <entitySelector>
                <entityClass>com.logistics.delivery.vrp.optaplanner.model.VrpVisit</entityClass>
                <filterClass>com.logistics.delivery.vrp.optaplanner.filter.GhostVisitSelectionFilter</filterClass>
            </entitySelector>
            <valueSelector variableName="splitRatio"/>
        </changeMoveSelector>
    </unionMoveSelector>

    <neighborhoodProviderClass>com.logistics.delivery.vrp.optaplanner.move.VrpNeighborhoodProvider</neighborhoodProviderClass>

    <acceptor>
        <acceptorType>VALUE_TABU</acceptorType>
        <entityTabuSize>10</entityTabuSize>
    </acceptor>
    <forager>
        <acceptedCountLimit>1000</acceptedCountLimit>
    </forager>
</localSearch>

<!-- 阶段二：延迟接受精修软约束 -->
<localSearch>
    <unionMoveSelector>
        <listChangeMoveSelector/>
        <listSwapMoveSelector/>
        <subListChangeMoveSelector/>
        <subListSwapMoveSelector/>
        <kOptListMoveSelector/>

        <changeMoveSelector>
            <entitySelector>
                <entityClass>com.logistics.delivery.vrp.optaplanner.model.VrpVisit</entityClass>
                <filterClass>com.logistics.delivery.vrp.optaplanner.filter.GhostVisitSelectionFilter</filterClass>
            </entitySelector>
            <valueSelector variableName="splitRatio"/>
        </changeMoveSelector>
    </unionMoveSelector>

    <neighborhoodProviderClass>com.logistics.delivery.vrp.optaplanner.move.VrpNeighborhoodProvider</neighborhoodProviderClass>

    <acceptor>
        <acceptorType>LATE_ACCEPTANCE</acceptorType>
        <lateAcceptanceSize>1000</lateAcceptanceSize>
    </acceptor>
    <forager>
        <acceptedCountLimit>2000</acceptedCountLimit>
    </forager>
</localSearch>

` Or via Java API / constraint streams.

Observation:

The error occurs when the list size is exactly 50, and the code tries to access index 50 (which is out of bounds, as valid indices are 0–49)

This suggests an off-by-one error in how DefaultUniqueRandomIterator or ElementAwareArrayList handles the upper bound when generating random indices for custom neighborhoods

The error appears during hasNext() / next() iteration over random moves

Question:
Is this a known bug with BiRandomMoveIterator and custom neighborhoods when the collection size is equal to the requested index? Is there a workaround or fix available? Should I avoid certain patterns in custom neighborhood definitions?

Thank you.

Please fill in:

Exact Timefold version

Whether you're using XML config, Java API, or constraint streams

Minimal code/config snippet of your custom neighborhood (if possible)

[triceo]

Thanks for reporting!

This has likely been solved already (see f52d936) and will be released in Solver 2.2.0 next week or so.